(FIELD OF THE INVENTION)
The present invention relates to a molten steel pouring nozzle which permits effective prevention of a reduction or a clogging of a bore of the nozzle, through which molten steel flows, when continuously casting an aluminum-killed molten steel containing aluminum.
(RELATED ART STATEMENT)
Continuous casting of molten steel is carried out, for example, by pouring molten steel received into a tundish from a ladle, through a molten steel pouring nozzle secured to a bottom wall of the tundish, into a vertical mold arranged below the molten steel pouring nozzle, to form a cast steel strand, and continuously withdrawing the thus formed cast steel strand as a long strand from the vertical mold.
As the above-mentioned molten steel pouring nozzle, a nozzle comprising an alumina-graphite refractory is widely used in general.
However, the molten steel pouring nozzle comprising an alumina-graphite refractory has the following problems:
When casting an aluminum-killed molten steel, aluminum added as a deoxidizer reacts with oxygen present in molten steel to produce non-metallic inclusions such as .alpha.-alumina. The thus produced non-metallic inclusions such as .alpha.-alumina adhere and accumulate onto the surface of a bore of the molten steel pouring nozzle, through which molten steel flows, to clog up the bore, thus making it difficult to achieve a stable casting. Furthermore, the non-metallic inclusions such as .alpha.-alumina thus accumulated onto the surface of the bore, peel off or fall down, and are entangled into a cast steel strand, thus degrading the quality of the cast steel strand.
For the purpose of preventing the above-mentioned reduction or clogging of the bore of the molten steel pouring nozzle caused by the non-metallic inclusions such as .alpha.-alumina present in molten steel, there is popularly used a method which comprises ejecting an inert gas from the surface of the bore of the molten steel pouring nozzle toward molten steel flowing through the bore, to prevent the non-metallic inclusions such as .alpha.-alumina present in molten steel from adhering and accumulating onto the surface of the bore.
However, the above-mentioned method comprising ejecting an inert gas from the surface of the bore of the molten steel pouring nozzle toward molten steel flowing through the bore, has the following problems:
A larger amount of the ejected inert gas causes entanglement of bubbles produced by the inert gas into the cast steel strand, resulting in the production of defects such as pinholes in a steel product after the completion of the rolling. This problem is particularly serious in the casting of molten steel for a high-quality thin steel sheet. On the other hand, a smaller amount of the ejected inert gas causes adhesion and accumulation of the non-metallic inclusions such as .alpha.-alumina onto the surface of the bore of the molten steel pouring nozzle, thus causing a reduction or a clogging of the bore. In the casting of molten steel for a long period of time, a stable control of the amount of an inert gas ejected from the surface of the bore of the molten steel pouring nozzle becomes gradually more difficult, according as a structure of the refractory forming the molten steel pouring nozzle is deteriorated. As a result, the non-metallic inclusions such as .alpha.-alumina adhere and accumulate onto the surface of the bore of the molten steel pouring nozzle, thus causing a reduction or a clogging of the bore. Furthermore, in the casting of molten steel for a long period of time, furthermore, a local erosion of the surface of the bore of the molten steel pouring nozzle is considerably accelerated by the ejected inert gas. This makes it impossible to continue the ejection of the inert gas and may cause a rapid clogging of the bore.
With a view to preventing a reduction or a clogging of the bore of the molten steel pouring nozzle without the use of a mechanical means such as the ejection of an inert gas, there is disclosed in Japanese Patent Provisional Publication No. 57-71,860 published on May 4, 1982, a molten steel pouring nozzle formed of a refractory consisting essentially of:
(hereinafter referred to as the "prior art 1").
However, the above-mentioned molten steel pouring nozzle of the prior art 1 has the following problems:
It is true that calcium oxide (CaO) rapidly reacts with non-metallic inclusions such as .alpha.-alumina, which are produced through the reaction of aluminum added to molten steel as a deoxidizer with oxygen present in the molten steel, to produce low-melting-point compounds such as CaO.cndot.Al.sub.2 O.sub.3 and 3CaO.cndot.Al.sub.2 O.sub.3. Therefore, calcium oxide has a function of preventing the non-metallic inclusions such as .alpha.-alumina from adhering and accumulating onto the surface of the bore of the nozzle. However, calcium oxide, when present alone, violently reacts with water or moisture in the air even at a room temperature to produce calcium hydroxide (Ca(OH).sub.2), which easily disintegrates and tends to become powdery, thus easily causing deterioration of the structure of the molten steel pouring nozzle. Great care is therefore necessary for storing the molten steel pouring nozzle. In addition, because of a high thermal expansion coefficient of calcium oxide, a considerable thermal stress is produced in the interior of the molten steel pouring nozzle when calcium oxide is present alone and the molten steel pouring nozzle is subjected to a heating which causes a non-uniform temperature distribution, thus resulting in a lower thermal shock resistance of the molten steel pouring nozzle.
For the problems as described above, it is difficult to use a molten steel pouring nozzle made of a refractory, in which calcium oxide is present alone, for a long period of time for the continuous casting of molten steel.
Furthermore, with a view to preventing a reduction or a clogging of the bore of the molten steel pouring nozzle without the use of a mechanical means such as the ejection of an inert gas, there is disclosed in Japanese Patent Provisional Publication No. 64-40,154 published on Feb. 10, 1989, another molten steel pouring nozzle formed of a refractory consisting essentially of:
(hereinafter referred to as the "Prior art 2").
However, the above-mentioned molten steel pouring nozzle of the prior art 2 has the following problems:
For the purpose of overcoming the problems encountered in the prior art 1, in which calcium oxide is present alone, the molten steel pouring nozzle of the prior art 2 is formed of a refractory mainly comprising calcium zirconate. Therefore, it is true that contact of calcium oxide contained in calcium zirconate with the produced non-metallic inclusions such as .alpha.-alumina causes a rapid reaction between these components, thus producing low-melting-point compounds such as CaO.cndot.Al.sub.2 O.sub.3 and 3CaO.cndot.Al.sub.2 O.sub.3. On the other hand, since calcium oxide is not present alone, no deterioration of the structure of the molten steel pouring nozzle is caused. In the prior art 2, however, calcium oxide contained in calcium zirconate does not move sufficiently toward the surface of the bore of the molten steel pouring nozzle, through which molten steel flows, so that calcium oxide does not come into sufficient contact with the produced nonmetallic inclusions such as .alpha.-alumina. As a result, the production of low-melting-point compounds brought about by the reaction between calcium oxide and the non-metallic inclusions such as .alpha.-alumina is insufficient to effectively prevent adhesion and accumulation of the non-metallic inclusions such as .alpha.-alumina onto the surface of the bore of the molten steel pouring nozzle.
Finally, with a view to preventing a reduction or a clogging of the bore of the molten steel pouring nozzle without the use of a mechanical means such as the ejection of an inert gas, there is disclosed in Japanese Patent Provisional Publication No. 3-221,249 published on Sep. 30, 1991, further another molten steel pouring nozzle formed of a refractory consisting essentially of:
where, a content of calcium oxide in said zirconia clinker being within a range of from 8 to 35 weight parts relative to 100 weight parts of said zirconia clinker, PA2 where, a content of calcium oxide in said calcium silicate being within a range of from 4 to 54 weight parts relative to 100 weight parts of said calcium silicate PA2 where, a content of calcium oxide in said zirconia clinker being within a range of from 8 to 35 weight parts relative to 100 weight parts of said zirconia clinker, PA2 where, a content of calcium oxide in said calcium silicate being within a range of from 40 to 54 weight parts relative to 100 weight parts of said calcium silicate.
(hereinafter referred to as the "Prior art 3").
However, the above-mentioned molten steel pouring nozzle of the prior art 3 has the following problems:
Calcium oxide in each particle of zirconia clinker easily moves toward the surface of each particle of zirconia clinker under the effect of coexistence of zirconia clinker with calcium silicate, and aggregates there. However, when molten steel contains a large amount of alumina(Al.sub.2 O.sub.3), calcium oxide can not be supplied in an amount sufficient to react with alumina in molten steel. In addition, when a temperature of molten steel is low, and in particular, at the time when the casting of molten steel starts or just before the completion of casting of one charge, in which the remaining amount of molten steel becomes small, a temperature of a portion of the molten steel pouring nozzle, which comes into contact with molten steel, becomes lower so that the movement and the aggregation of calcium oxide in each particle of zirconia clinker toward and on the surface of each particle of zirconia clinker are decreased or discontinued, and as a result, an amount of calcium oxide to react with alumina in molten steel becomes insufficient. It is therefore impossible to effectively prevent adhesion and accumulation of the non-metallic inclusions such as .alpha.-alumina onto the surface of the bore of the molten steel pouring nozzle.
Under such circumstances, there is a strong demand for the development of a molten steel pouring nozzle which permits prevention of a reduction or a clogging of a bore of the nozzle and deterioration of the structure of a refractory forming the nozzle economically and for a long period of time without the use of a mechanical means such as the ejection of an inert gas, even when a temperature of molten steel is low or molten steel contains a large amount of non-metallic inclusions such as .alpha.-alumina, but such a molten steel pouring nozzle has not as yet been proposed.